A projection television system includes three individual cathode ray tubes (CRTs), each having a different screen phosphor corresponding to one of red, green or blue colors. Relatively high level red, green and blue video drive signals are coupled to respective electron guns of the CRTs from respective drive amplifiers or "drivers" which amplify respective relatively low level red, green and blue video signals produced by a signal processing section. Usually, the video drive signals are coupled to the cathodes of the CRTs. In response to the respective video drive signals, the CRTs generate electron beams which strike the respective screen phosphors causing them to emit light outputs of particular wavelengths corresponding to respective colors. The light outputs produced by the CRTs are transmitted by an optical system to a screen where a composite color image is reproduced.
The intensity of the light produced by a CRT is a function of the intensity of the electron beam, which in turn, is a function of the magnitude of the drive signal coupled to an electron gun. When the magnitude of a video signal coupled to a driver is high, it is expected that a corresponding high intensity light output will be produced by the CRT. However, the efficiency of a phosphor to convert beam current to light may be reduced or "roll-off" as the magnitude of the drive signal increases. This is particularly a problem with the blue phosphor. More specifically, the blue phosphor becomes less efficient than the green and red phosphors as the CRTs are driven increasingly hard. This nonuniformity is due to the doping required to get the blue phosphor compound to emit light at the desired wavelength.
The problem of the blue phosphor inefficiency or roll-off is manifested in two ways. The first relates to "white balance" or grey scale retention, and the second relates to color fidelity.
Considering the case of white balance first, it is very desirable for a color television system to maintain constant white balance or grey scale. When "white" representative red, green and blue video signals are coupled to the image reproducing device, it should produce a "white" image. White comes in a variety of tints. For example, the "white" light produced by an incandescent light bulb is visibly more yellow than the "white" light produced by a fluorescent light bulb. The tint of white can be quantified by what is referred to as the "color temperature". The color temperature can be adjusted by setting the ratio's of the magnitudes of the red, green and blue CRT beam currents. A common value for the color temperature is designated as 9300 degrees Kelvin and corresponds to the following percentages of the total drive current for the red (R), green (G) and blue (B) drive currents:
R=12.9% PA1 G=48.9% PA1 B=38.2%
If the phosphors of the three CRTs were uniformly efficient, maintaining these percentages would produce a "white" image of the specified constant color temperature as the magnitudes of the drive currents increased together. However, the nonuniformity discussed above results in a very undesirable yellowing of peak "white" portions of an image.
In the case of color fidelity, a uniform color scale is also to be expected. Hues (or tints) are produced by ratios of red, green, and blue currents different than those required to make white. A particular hue should remain constant as the magnitudes of the drive currents increase together. Certain hues are affected more than others by the blue phosphor inefficiency. Flesh tones are very critical. Blue phosphor roll-off causes flesh tones to shift toward green. Of course, this is very undesirable.
Prior solutions to the blue phosphor roll-off problem have involved circuitry for boosting the current in the blue CRT at the point at which the blue phosphor efficiency begins to roll off. Typically, a switchable gain element is added to the blue CRT driver to selectively increase the gain of the driver so as to correspondingly increase the drive current for the CRT. The additional current intensifies the electron beam and causes the blue phosphor to be driven harder, thus increasing the light output of the blue CRT. The additional blue light output restores the white balance or hue of the image. Viewed another way, the prescribed current ratios are altered at some input signal level in order to compensate for the blue phosphor roll-off.
FIGS. 1 and 2 show diagrams of a projection television system with respective prior blue roll-off compensation networks as are known to the present inventors. The common components of the arrangements shown in these figures will be described before the respective roll-off compensation networks are individually described.
In each of these figures, a video signal processing unit 10 provides relatively low level red (R), green (G) and blue (B) video signals which are coupled to CRT drivers 12R, 12G and 12B, respectively. CRT drivers 12R, 12G and 12B amplify and invert the respective low level video input signal to produce a relatively high level video output signal suitable for driving a respective one of red, green and blue CRTs 14R, 14G and 14B. The light outputs of red, green and blue CRTs 14R, 14G and 14B are guided by an optical system (not shown) to a screen (not shown) where they are combined to produce a composite image.
With the exception of a blue roll-off compensation network, CRT drivers 12R, 12G and 12B are substantially identical, and accordingly only blue CRT driver 12B will be described in detail. Blue CRT driver 12B is shown as simply comprising a transistor QD in a common emitter amplifier configuration, although it will be appreciated that CRT drivers are in practice usually more complex. A relatively low level blue video input voltage VIN is coupled to the base of transistor QD and an inverted, relatively high level video output voltage VOUT is produced at the collector of transistor QD. The emitter of transistor QD is coupled to signal ground through an emitter resistor RE1 and its collector is coupled to a source of relatively high voltage B+ through a collector resistor RC. The collector of transistor QD is coupled to the cathode of a blue CRT 14B through a resistor RK. As video output voltage VOUT decreases from a blanking or black (BLANK) level, the beam current and the blue light output of blue CRT 14B increase. A network including the series combination of a resistor RE2, a variable resistor RE3 and an adjustable source of a bias voltage VBIAS is coupled parallel with emitter resistor RE1. Variable resistor RE2 is adjusted to adjust the gain of the driver 12B and is used in the color temperature adjustment. Bias voltage VBIAS is adjusted so that driver 12B and therefore CRT 14B are cutoff at the blanking (BLANK) level of the video input voltage VIN.
The blue roll-off compensation networks shown in FIGS. 1 and 2 each operate by increasing the emitter current of transistor QD, thereby correspondingly increasing the collector current, decreasing output voltage VOUT, increasing the blue beam current, and increasing the blue light output, at a magnitude of input voltage VIN which corresponds to the point at which the blue phosphor becomes relatively less efficient.
In the arrangement shown in FIG. 1, the blue roll-off compensation network comprises the series combination of a diode CRB and resistor RB connected in parallel with resistor RE2 of the emitter circuit of transistor QD. In this configuration, as the current flowing through resistor RE2 increases, due to increases of input voltage VIN, the voltage across resistor RE2 increases and will eventually become sufficient to cause diode CRB to conduct. As a result, the emitter current of transistor QD is increased (i.e., "boosted") by the current flowing through the series combination of diode CRB and resistor RB. The collector current of transistor QD is correspondingly increased, output voltage VOUT is decreased and the blue beam current is increased. This provides additional stimulus to the blue phosphor so as to correct for the roll-off.
The roll-off compensation network shown in FIG. 1 can be modified by adding one or more additional diodes in series with diode CRB and poled in the same sense. The roll-off compensation network shown in FIG. 1 can also be modified by utilizing a Zener diode poled in the opposite sense as diode CRB.
In the type of roll-off compensation network described with respect to FIG. 1, the point at which compensation will occur is primarily determined by the number of diodes which are used, and if a Zener diode is used, the Zener voltage; and the level of the current boost is primarily determined by the value of resistor RB.
In the arrangement shown in FIG. 2, the blue roll-off compensation network comprises the series combination of a diode CRB, a resistor RB and a source of bias voltage VBOOST connected in parallel with resistor RE1 of the emitter circuit of transistor QD. In this configuration, diode CRB will eventually be causes to conduct as the voltage across resistor RE1 increases due to increases of input voltage VIN. As a result, the emitter current of transistor QD is increased (i.e., "boosted") by the current flowing through the series combination of diode CRB and resistor RB. The collector current of transistor QD is correspondingly increased, output voltage VOUT is decreased, the blue beam current is increased, and additional stimulus is provided to the blue phosphor so as to correct for the roll-off.
In the type of roll-off compensation network described with respect to FIG. 2, the point at which compensation will occur is primarily determined by the magnitude of bias voltage VBOOST; and the level of the current boost is primarily determined by the value of resistor RB.
The adjustment of variable RE3 affects the amount of boost provided by the type of roll-off compensation network discussed with respect to FIG. 1, whereas it does not affect the amount of boost provided by the type of roll-off compensation network discussed with respect to FIG. 2. It is desirable for the adjustment of variable resistor RE3 to affect the gain of the roll-off compensation network for the following reasons.
Gain controlling variable resistor RE3 of each of drivers 12R, 12G and 12B is adjusted to provide the proper color temperature. Variable resistor RE3 is initially set to zero ohms. Increasing the resistance of variable resistor RE3 decreases the gain. For purposes of the color temperature adjustment, red, green and blue video signals corresponding to a completely white image (a so called 100 IRE full field image) are applied to drivers 12R, 12G and 12B. At this point, the automatic beam current limiting network (not shown) of the television system operates to limit the beam currents of CRTs 14R, 14G and 14B. Automatic beam current limiting networks typically included in television systems operate by sensing the average current drawn from the CRT high voltage power supply, and by reducing the red, green and blue beam currents by reducing the gains of the respective channels when the sensed current exceeds a predetermined threshold. If the resistance of variable resistor RE3 for blue driver 12B needs to be increased to decrease the gain of blue driver 12B in order achieve the proper color temperature when the television system is operating to produce a 100 IRE full field image, it is likely that less boost will be needed for this condition than in the condition in which variable resistor RE3 does not need to be adjusted. Therefore, it is desirable for the blue roll-off compensation network to track the adjustment of variable resistor RE3, as is the case for the type of roll-off compensation network discussed with respect to FIG. 1.
The type of roll-off compensation network discussed with respect to FIG. 1 is also more advantageous than the type of roll-off compensation network discussed with respect to FIG. 2 with regard to the DC output conditions of video signal processing unit 10, as will now be discussed.
Video signal processing unit 10 usually includes a video processing integrated (IC) which produces relatively large and unpredictable DC offsets at its R, G and B outputs. The DC offsets are coupled to the bases of transistors QD of respective drivers 12R, 12G and 12B and make it desirable for bias voltage VBIAS for each of drivers 12R, 12G and 12B to be adjustable to compensate for the respective offsets. In the type of blue roll-off compensation network described with respect to FIG. 1, in which the threshold element is coupled between the emitter of transistor QD and the source of variable bias voltage VBIAS, the boost threshold is not affected by the blue channel DC offset of video signal processing unit 10 because the DC offset is cancelled by adjustable bias voltage VBIAS. However, in the type of the roll-off compensation network described with respect to FIG. 2, in which the threshold element is coupled between the emitter of transistor QD and the source of bias voltage VBOOST, if the source of bias voltage VBOOST is not adjustable, the boost threshold will vary inversely with the DC offset at the B output of video processing unit 10. Making bias voltage VBIAS adjustable will solve this problem. However, an adjustable source of boost threshold bias voltage VBOOST will increase the cost of the system. Moreover, the the level of bias voltage VBOOST necessary to compensate for the DC offset will probably not correspond to the needed boost threshold.
While, the type of roll-off compensation network discussed with respect to FIG. 1, has advantages over the type of roll-off compensation network discussed with respect to FIG. 2, it also has some inherent disadvantages. In the case of the type of network discussed with respect to FIG. 1, the diode or diodes offer only discrete breakpoints for the boost threshold, roughly at integer multiples of the forward voltage drop of a diode. None of the discrete breakpoints may coincide with the point at which the phosphor inefficiency begins. In addition, the temperature sensitivity of the arrangement is a function of the number of diodes used.
With regard to the modification of the arrangement shown in FIG. 1 in which a Zener diode is employed, it is noted that a low voltage Zener diode is required. Such Zener diodes have an "on" voltage which is very much a function of the current through it. This results in an unpredictable boost threshold.
Both types of roll-off compensation networks discussed with respect to FIGS. 1 and 2 have the inherent problem that the boost threshold and the boost gain are relatively dependent on one another because the components which determine these characteristics are connected in series with one another. Further, the capacitance of a diode type of switching arrangement may adversely effect the transient response of the blue channel compared to the red and green channels. With regard to the modification of the arrangement shown in FIG. 1 in which a Zener diode is employed, it is noted that the capacitive effects of Zener diodes tend to be more significant than those of ordinary diodes. Still further, the leakage current of a diode may cause current boosting when none is wanted.
Thus, there is a need in the art for a roll-off compensation network which provides as many desirable aspects of the arrangements described with respect to FIGS. 1 and 2 and which avoids as many of their deficiencies as possible.